The tower that would be San Francisco's tallest structure is scheduled to go to the Planning Commission for its final approvals next month, five full years after being unveiled.

The obelisk-like building that would rise at First and Mission streets also has been tweaked in two key ways since an updated design was introduced in March after several years in limbo because of the economy.

This month's revision of the revision adds highly visible connections to the rooftop park planned for the new Transbay Terminal being built next door. In addition, the crown of the 1,070-foot-tall tower has been altered to give it a more distinctive look.

"They responded very directly to concerns about the top and the base," said John Rahaim, city planning director. "These are simple things, but I think they make a big difference."

For their part, the developers and architects describe the changes as part of the natural fine-tuning of a building intended from the start to have a major civic presence.

This is especially true of the tower's crown, which is designed to extend 150 feet beyond the highest occupied floor. It also will top off the skyline; the Transamerica Pyramid, downtown's tallest building since it opened in 1972, is 853 feet high.

The latest design

Prior versions of the tower treated the crown as an open-air extension of the outer walls of the 60 floors below, with horizontal bands of glass and steel continuing upward even as they thinned out to allow light to pass through. The newest design adds a deep vertical slit that runs up each side of the crown, backed by perforated metal that will be lit at night for a sculptural effect.

"We wanted something visible at urban scale but almost Zen-like in its simplicity," said architect Fred Clarke of Pelli Clarke Pelli, who called the redone top a "transformative refinement ... the idea is to further lengthen and slim the profile and also create something more distinctly emblematic of the city."

The changes in the sky are aesthetic. The ones on the ground respond to concerns that the tower would close off the public even though it will be built on what now is publicly owned land.

Also a half-acre plaza

In addition to the tower, the project includes a half-acre plaza to the east that will be built and maintained by the developer, Hines. To the south is the main entrance to the new transit center, which when it opens in 2017 will rise 80 feet and be topped by a 5.4-acre park.

Hines' and Pelli Clarke Pelli's winning entry in the competition for the Transbay site included not only the tower and the park-topped transit system, but a diagonal elevator ascending from the plaza to link the two spaces.

That connection was missing in the plans presented to the city in March, as was any public access to the tower beyond ground-floor retail space. Now, though, a funicular elevator will glide upward to the park through a landscape of 20 redwood trees spaced 30 feet apart, part of the plaza design by PWP Landscape Architecture.

Another path to the rooftop park will be through a seating-filled public lobby added to a corner of the tower facing the plaza. In that lobby will be an express elevator to the fifth floor, where the tower and the park will be connected by a 128-foot-wide deck lined on the tower side with shops.

City planning hearing

The Planning Commission hearing is scheduled for Oct. 18, 11 weeks after Mayor Ed Lee signed into law a plan for what now is called the Transit Center District. That plan raises heights on several sites to allow extra-tall towers that would generate money for the rebuilt transit terminal through fees and land-sale proceeds.

The plan also included the environmental study for the Hines tower - so if the Planning Commission gives its blessing, no regulatory hurdles remain.

"If everything works out perfectly, we could start (construction) in the late summer of 2013," said Paul Paradis, a senior managing director at Hines.

The project will need financial backing as well as planning approvals to proceed. Hines' primary financial partner, MetLife, pulled out of the project this summer. Hines has a Sept. 30 deadline to pay $185 million for the tower site to the Transbay Joint Powers Authority, which is building the transit center.

Asked if the $185 million obligation will be settled before the Planning Commission meeting, Paradis said Hines does not comment on financial negotiations, but "we're comfortable with where we are" in terms of wrapping up the purchase.